Aqueous fluids, such as waste water, acid mine tailings, groundwater, and the like, can have high concentrations of various ions which can pose significant environmental and health hazards. Thus, removal of ions from aqueous fluids can be required by statute, rule, or regulation before disposal or use of such aqueous fluids.
Dissolved ions can be precipitated from aqueous fluids as ionic compounds, which can then be separated from the aqueous fluids. One class of ionic compounds that can be precipitated is ettringites. Ettringites, which are commercially useful components of cements, such as Portland cement, are hydrous calcium aluminum sulfate minerals, which can include a mineral with the chemical formula Ca6Al2(SO4)3(OH)12.26H2O and a series of chemically analogous isostructural compounds. For instance, all or part of the aluminum can be substituted with one or more trivalent metal cations such as Fe (III), Mn(III), and Cr (III), all or part of the calcium can be substituted with one or more divalent metals, such as Mg2+, all or part of the hydroxide can be substituted with one or more monovalent anions ions, such as chloride, fluoride, iodide, and bromide, and all or part of the sulfate can be substituted with one or more other oxyanions, such as chromate, selenate, borate, and carbonate. As an example, a boron containing ettringite can have a borate oxyanion in place of one or more sulfate anions. Also, the amount of water of hydration may vary somewhat.
Although the general definition of ettringite discussed above is often accepted, ettringites can also be defined more specifically as compounds with the formula Ca6M2(OH)x(A)y(D)z.E(H2O), wherein M is one or more trivalent metals, A is one or more oxyanions with one or more charges n−, D is one or more monovalent anions with charge 1−, E is 26 or 32, n is 1, 2, 3, or 4, and x, y, and z are defined such that x+(y·n)+z=18, x>0, y>0, and z≧0. When an ettringite contains more than one oxyanion A with different charges n-, then (y)(n) is the sum of the value y for each oxyanion A multiplied by the n values for each respective oxyanion. For example, if A is (VO4)(ClO3)3, wherein VO4 has a charge of 3-(n=3) and ClO3 has a charge of 1-(n=1), then (y)(n) is (1)(3)+(3)(1)=6. The values of x, y, and z are often integers, although this is not required unless otherwise specified. The values of x, y, and z can be defined such that z is 0, x is 12, and (y·n) is 6, although this is also not required unless otherwise specified.
Examples of the one or more oxyanions A with a charge n− include one or more of sulfate, chromate, carbonate, sulfite thiosulfate, selenate, selenite, molybdate, silicate, vanadate, arsenate, nitrite nitrate manganate, borate, iodite, iodate, formate, acetate, and propionate. Examples of monovalent anions with charge 1− are hydroxide, chloride, fluoride, bromide, and iodide. Examples of trivalent metals are Fe(III), Mn(III), Ti(III), and Cr(III).
One or more ettringites can be formed by adding one or more bases and one or more aluminum salts to an aqueous fluid with dissolved oxyanions A. One or more trivalent metals M can also be in dissolved in the aqueous fluid, but this is not required since the aluminum salt can provide aluminum as the one or more trivalent metals M. Similarly, the ettringite calcium, or one or more divalent metals that replace all or part of the calcium, can be dissolved in the aqueous fluid or added as part of the one or more bases (which can be one or more of ammonium hydroxide, calcium oxide, calcium hydroxide, sodium hydroxide, magnesium hydroxide, lime, and slaked lime), and must be present in excess to form a precipitate. Under these conditions, ettringite can precipitate from the aqueous fluid, thereby decreasing the concentration of one or more of the ions that constitute the ettringite structure.
A serious drawback to these procedures is that the addition of one or more aluminum salts and one or more bases to the aqueous fluid adds additional anions to the aqueous fluid. For example, adding aluminum chloride as the aluminum salt and sodium hydroxide as the base increases the levels of chloride and sodium ions in the aqueous fluid. These additional ions often have to be removed from the aqueous fluid to make the aqueous fluid suitable for reuse or disposal. When the base is a liquid, such as slaked lime, the volume of the aqueous fluid can increase significantly upon addition of the liquid base, which adds to the amount of aqueous liquid that must be processed and disposed. When one or more aluminum salts are used, a stoichiometric excess of aluminum must be added, and not all of the aluminum in the one or more aluminum salts precipitate as part of the one or more ettringites. Often, this excess aluminum also has to be removed from the aqueous fluid. Another problem arises when the one or more ions to be removed from the aqueous fluid comprise sulfate. Ettringites comprising sulfate can have an aluminum to sulfate molar ratio if about 1 to about 1.5, or from about one to between about 1.5 and about 2. Although it is possible in theory to precipitate more than 2 mols of sulfate per mol of aluminum, this has not been reported. Since aluminum salts can be expensive, higher sulfate to aluminum ratios would be advantageous.